1. Field of the Prior Art
The present invention relates to ferroelectric liquid crystal compositions. More particularly, the present invention relates to ferroelectric liquid crystal compositions having a quick response property wherein the phase transition series assumes an isotropic liquid.fwdarw.a cholesteric phase.fwdarw.a chiral smectic C phase in the named order from the high temperature side without assuming a smectic phase, and to switching elements using the same.
2. Description of the Prior Art
Liquid crystal compounds are widely used as materials in display elements, and most of these liquid crystal elements are of the TN type display system, and the liquid crystal materials are in the nematic phase.
The TN type display system has advantages such as moderated eye fatigue and extremely small consumption of electric power because of being a non-emissive type, whereas it has disadvantages such as slow response and disappearance of display at certain visual angles.
In recent years, this system is being improved in such ways as to keep the characteristics of flat displays, and particularly, a faster response and the enlargement of the visual angle are demanded.
In order to meet these demands, improvements in liquid crystal materials have been attempted. However, as compared with other emissive type displays (e.g., electroluminescence displays and plasma displays, it is apparent that the TN type display has a much slower response time and has a smaller visual angle.
In order that characteristics of the liquid display element such as features of the non-emissive type and small consumption of electric power may be maintained and in order that a quick response corresponding to that of the emissive type displays may be assured, it is essential to develop a novel liquid display system in place of the TN type display system.
In one of such attempts, a display system in which the optical switching phenomenon of ferroelectric liquid crystals is utilized has been suggested by N.A. Clark and S. T. Lagewall (see Appl. Phys. Lett., 36, p 899, 1980).
The presence of the ferroelectric liquid crystals was announced for the first time in 1975 by R. B. Mayer et al. (see J. Phys., 36, p 69, 1975), and from the view of structure, these crystals belong to the chiral smectic C phase, the chiral smectic I phase, the chiral smectic F phase, the chiral smectic G phase, and the chiral smectic H phase (hereinafter referred to simply as "S.sub.C *", "S.sub.I *", "S.sub.F *", "S.sub.G ", and "S.sub.H *", respectively).
In the chiral smectic phase, molecules are in layers and inclined with respect to the surface of the layer, and the helical axis is vertical to this layer surface.
In the chiral smectic phase, spontaneous polarization takes place, and therefore, when a DC electric field is applied to these layers in parallel with the layers, the molecules turn around the helical axis in accordance with its polarity. The display element of ferroelectric liquid crystals utilizes this switching phenomenon.
Nowadays, of the chiral smectic phases, much attention is particularly paid to the S.sub.C * phase.
The display system in which switching phenomenon of the S.sub.C * phase is utilized can be further classified into two types: a birefringence type system using two polarizers and a guest/host type system using a dichroic dye.
Features of these display systems are:
(1) Response time is very short. PA1 (2) Memory properties are present. PA1 (3) Display performance is not affected by the visual angle. PA1 (1) the layers deform into the shape of the letter L (the resulting shape is called the chevron structure), and therefore a zigzag defect is formed; PA1 (2) the molecules adopt a splayed arrangement, and therefore complete memory properties cannot be obtained; and PA1 (3) in order to secure memory properties, it is required to make the thickness of the cell 2 .mu.m or less, but mass production of such a cell is difficult under the present fabrication technique. In particular, the problems under (1) and (2) must be solved for displays utilizing ferroelectric liquid materials as display elements in order to prevent deterioration of display quality. PA1 and compounds of the formula (II): ##STR8## wherein R.sup.3 and R.sup.4, which may the same or different, represent each an alkyl group or an alkoxy group having 1 to 18 carbon atoms; PA1 a component B of a compound of the formula (III): ##STR9## wherein R.sup.5 represents an alkyl group or alkoxy group having 1 to 18 carbon atoms, and * indicates an asymmetric carbon atom; PA1 a component C of a compound of the formula (IV): ##STR10## wherein R.sup.6 represents an alkyl group or alkoxy group having 1 to 18 carbon atoms, X represents ##STR11## Y represents a hydrogen atom or a halogen atom, and * indicates an asymmetric carbon atom; and PA1 a component D of a compound of the formula (V): ##STR12## wherein R.sup.7 represents an alkyl group or alkoxy group having 1 to 18 carbon atoms, n is an integer of 0 to 10, Z represents a hydrogen taom or a halogen atom, and indicates an asymmetric carbon atom, PA1 the proportion of said component A being 20 to 80 wt. %, that of the component B being 5 to 30 wt. %, that of the component C being 3 to 10 wt. %, and that of the component D being 3 to 20 wt. %, preferably the proportion of said component A being 50 to 75 wt. %, that of the component B being 10 to 30 wt. %, that of the component C being 3 to 7 wt. %, and that of the component D being 3 to 20 wt. %, based on the total amount of said four components A, B, C, and D, and the phase transition series assumes an isotropic liquid.fwdarw.a cholesteric phase a chiral smectic C phase in the named order from the high temperature side. PA1 with the amount of the component E being 3 to 10 wt. %, based on the total amount of said components A, B, C, and D, and the phase transition series of said composition assumes an order of isotropic liquid.fwdarw.cholesteric phase.fwdarw.chiral smectic C phase, starting from the high temperature side.
Thus, the display systems have the possibility of achieving the high-density display and is considered to be effectively utilizable in display elements. However, also in these display systems, there are many problems to be solved.
The display systems that use the switching phenomenon of the S.sub.C * phase are accompanied by problems, for example, as follows:
However, recently, C. Bowry et al. have proposed a new idea to solve the above problems [see Euro Display 87, 33 (1987)].
They state that when a cell to which SiO had been deposited obliquely is used, and as the S.sub.C * material, use is made of a material whose phase transition series is the I.sub.SO .fwdarw.N*.fwdarw.S.sub.C * (wherein I.sub.SO stands for an isotropic liquid, and N* stands for a cholesteric phase), the above structure having the shape of the letter L can be obviated, so that the zigzag defect is hardly recognized. According to their idea, the use of a cell having an obliquely deposited thin film has brought about good memory properties.
Therefore, recently, the study of display elements using ferroelectric liquid crystal materials of this system has become very popular.
However, there are few practical ferroelectric liquid crystal materials having an I.sub.SO .fwdarw.N*.fwdarw.S.sub.C * type phase transition series. For example, since ferroelectric liquid crystal compositions for guest/host type display elements disclosed in Japanese Patent Laid-Open Publication No. 22889/1987 that was filed by the present applicants takes on an I.sub.SO .fwdarw.N*.fwdarw.S.sub.C * type phase transition series, it can be used in the system proposed by C. Bowry et al.
However, the response time of the ferroelectric liquid crystal compositions disclosed in Japanese Patent Laid-Open Publication No. 22889/1987 is very long (for example in the case of a ferroelectric liquid crystal composition disclosed in Example 4), and is not practical.
Consequently, further improvement in the responsiveness is eagerly demanded.